public static System.Drawing.Image AddWatermarkImage(string filename)
    {
        string text = "S? LAO ??NG TH??NG BINH H?I D??NG";
        System.Drawing.Font font = new System.Drawing.Font("Arial", 20, FontStyle.Bold);

        using ( System.Drawing.Image image = System.Drawing.Image.FromFile(filename) )
        using ( Graphics g = Graphics.FromImage(image) )
        {
            g.TranslateTransform(image.Width / 2, image.Height / 2);
            g.RotateTransform(45);
            SizeF textSize = g.MeasureString(text, font);
            g.DrawString(text, font, Brushes.Red, -( textSize.Width / 2 ), -( textSize.Height / 2 ));

            return image;
        }


    }
    public static void WaterMarkImage(string folderPath)
    {
        bool exists = Directory.Exists(folderPath);
        if ( exists )
        {
            DirectoryInfo d = new DirectoryInfo(folderPath);
            FileInfo[] Files = d.GetFiles(); 

            if ( Files.Length > 0 )
                foreach ( var img in Files )
                {
                    using ( System.Drawing.Image imgOut = AddWatermarkImage(img.FullName) )
                    {
                        imgOut.Save(img.FullName, ImageFormat.Jpeg);
                    }

                }
        }

    }